The entrance to a mine tunnel (as opposed to a shaft). Usually a large opening supported by wood or metal beams, but can vary in size and construction. The walls, cieling, and supports may or may not be unstable based on age, climate, and the type of rock or soil it enters. Some adits have ore cart rails leading inside, and some are blocked by metal gates installed by the mine owner or local governments.
From the Glossary of Mining Terms (http://www.miningindia.com/glossary/): "An opening driven horizontally into the side of a mountain or hill for providing access to a mineral deposit. Strictly speaking, an adit is open to the atmosphere at one end, a tunnel at both ends."
Added by Akron:Sometimes referred to as a Drift, or Portal. Also an opening in the face of a dam for access to galleries or operating chambers.